A user account. Most Linux system are set up with at least one ordinary user (you) and an administrator = root.

session

When you log in to your system, an application or some server you start a session, which lasts until you log out of it again.

logon

The act of starting a new session on a system. This usually requires a username and a password. Several users might be logged on to a system simultaneously, although only one would be active at a time.

logout

The act of ending a session on a system. The system will then wait for another logon rather that just shutting down.

desktop

The workspace you see, when you log in. It typically has a panel with a start menu and other widgets. The workspace itself can also hold various widgets.

Technology that allows applications to offer users the option to easily download and install plugins, artwork, scripts or similar additions using a unified interface. Upload for data sharing, however, is done less easily.